The present invention relates to a process for producing a cholesteric liquid crystal polyester film or sheet having a high strength, a high modulus and a good dimensional stability.
Recently, with the reduction in size and thickness of magnetic tapes and discs, there has been an increasing demand for a film or sheet superior in strength, modulus and dimensional stability.
In order to obtain a thin film or sheet having a high strength and a high modulus it is necessary to orient a polymer molecular chain biaxially, and the method adopted most commonly for realizing such biaxial orientation is a biaxial drawing. However, biaxial drawing is not only costly high but also encounters a limit in attainable strength and modulus because a biaxial drawing condition is rather restricted. Also in point of dimensional accuracy a drawn film is prone to shrink on heating.
One means for overcoming the above problems that has been proposed is a biaxially self-reinforced cholesteric liquid crystal polymer (U.S. Pat. No. 4,412,059).
In a cholesteric liquid crystal polymer, a nematic liquid crystal layer is induced by an optically active unit to form a helical structure twisted at a certain angle. In the case of a cholesteric liquid crystal polymer having a rigid chain and an optically active unit copolymerized in the main chain, it is expected that there will be obtained a film or sheet which is self-reinforced biaxially like a laminate.
However, in a cholesteric liquid crystal polymer, unlike a low molecular liquid crystal, orientation is so slow that it is rarely the case that the entire film or sheet is composed of an ordered planar texture in which all the cholesteric helical axes are perpendicular to a plane (monodomain state). Usually, a cholesteric polymer film or sheet is composed of a large number of small planar texture (polydomain structure) similar to a polycrystal structure (see Tadahiro Asada, "Kobunshi Kako," 32, 79 (1983), 33, 119 (1983)).
Actually, in thermotropic cholesteric liquid crystal polyesters with high molecular weight, it is not easy to observe such a texture as oily streaks or a fingerprint pattern when they are in a molten condition. Further, there has been no reporting of an ordered planar texture (monodomain structure) when these liquid crystal polyesters are frozen in a film or sheet.
In a film of a polydomain structure, the anisotropy of mechanical properties caused by the flow during molding is reduced, but since mechanical properties are governed by tiemolecules between domains, it is impossible to make the most of the high strength and high modulus of liquid crystal polymers. On the other hand, in the case of a film having a monodomain structure or a structure similar thereto, because of a layer structure parallel to a plane throughout the whole surface of the film, not only is the anisotropy of mechanical properties reduced but also it is possible to realize the high strength and high modulus peculiar to liquid crystal polymers.
Therefore, the development of a technique for attaining a monodomain structure of cholesteric liquid crystal polyesters has been desired.